1. Field of the Invention
The present invention relates to a sequential burst mode activation circuit. More particularly, the present invention provides a circuit topology for improving the consistency of performance in the activation and intensity variation of multiple loads. The present invention has general utility wherever multiple loads are employed for intensity variation. Further, the present invention has specific utility where multiple fluorescent lamps, especially multiple cold cathode fluorescent lamps (CCFLs), are employed, for example, in television and computer screens, and in backlights for LCDs (Liquid Crystal Displays).
2. Description of Related Art
Various lighting and dimming circuits and techniques for lighting or dimming lamps or varying intensities of loads are known. One method of dimming a fluorescent lamp, especially as used in a backlight of a liquid crystal display (LCD), is known as a voltage controlled dimming system. The voltage controlled dimming system includes current control and current feedback control. According to the voltage control dimming system, dimming is performed by varying an input voltage to an inverter so as to adjust an output voltage from the inverter (i.e., an application voltage to the fluorescent tube). As the fluorescent tube emits light using discharging energy, when the application voltage to the fluorescent tube is too low, the discharging becomes unstable. For this reason, a large dimming range cannot be achieved by the voltage control dimming system, and the possible dimming ratio is only around 2:1, the dimming ratio being indicative of the dimming range of the lamp system.
Another technique for dimming a fluorescent lamp is the xe2x80x9cburst modexe2x80x9d dimming system in which an alternating signal that is supplying power to the lamp is cut with a notch of variable width so as to reduce the power applied to the lamp and thereby provide the desired dimming. The smaller the widths of AC power provided to the lamp, the lower the luminance at which the lamp operates. A common device for providing the ability to vary the width of the pulses are commercially-available pulse-width modulators (xe2x80x9cPWMxe2x80x9d).
In burst mode dimming, dimming is performed by periodically flashing the light source with a varying time ratio between the light-on duration and light-out duration. Therefore, this system, as opposed to the aforementioned voltage controlled dimming method, offers a large dimming ratio, potentially greater than 100:1, thereby allowing for large variations in luminosity.
U.S. Pat. No. 5,844,540 provides lighting/dimming circuitry for the back light control function in an LCD (Liquid Crystal Display). A xe2x80x9cPWM dimmer driving circuitxe2x80x9d modulates the magnitude of current to be supplied through an inverter to a fluorescent tube on the back surface of a liquid crystal panel. One goal of this circuitry is to prevent inconsistency of lighting, or occurrence of flicker, between the back light, or fluorescent tube, and the LCD; the other goal is to reduce sound noise. The PWM and inverter circuitry modulates the light source driving means so as to have the ability to periodically flash the light source with varying time ratios between the light on and light off durations, thereby creating different average intensities of light. The light-on duration is determined by a xe2x80x98pulse count circuitxe2x80x99 which provides an input for the PWM circuitry; this pulse count circuit counts the number of pulses of the LCD panel horizontal synchronizing signal, and provides for an on-duration that allows for the back light to synchronize it""s lighting signal with that of the LCD. Further, the lighting/on-off frequency of the light source is a division of the horizontal driving frequency of the LCD panel""s horizontal synchronizing signal, thereby allowing both LCD panel""s display and the back light to be in phase with each other. This topology provides a xe2x80x9cburst-modexe2x80x9d dimming system but only for a single fluorescent lamp. It further advocates synchronization of backlight lighting with that of the LCD in order to prevent inconsistency of lighting between the LCD and backlight. Note that fluorescent lamps, especially cold cathode fluorescent lamps, are high in impedance when initially powered up. If multiple CCFLs (cold cathode fluorescent lamps) were utilized, synchronization of all lamps with one light source would result in current ripples; these current ripples retard inverter performance and cause flicker. This is because, where multiple CCFLs are synchronized, a power supply needs to provide enough power to turn on all CCFLs concurrently. The instant power delivered from the power supply causes the supply voltage to drop due to its limited dynamic response. Therefore, the use of PWM signals, i.e. xe2x80x9cburst-modexe2x80x9d dimming, is not, by itself, effective in providing a solution to flicker/noise in multiple lamp configurations.
One technique used to compensate for flicker or noise in the burst-mode dimming of multiple CCFLs is to place a capacitor in series with the power supply to absorb power surges that cause the current ripples. A drawback of this technique is that, when the lamps turn off in each burst mode cycle, the power supply line, which has an intrinsic inductance, continues to carry current which charges the capacitor, yielding an increase in output voltage.
Prior art teachings with the activation with multiple loads, where the loads are not fluorescent lamps, do not address the flicker or noise problem presented by the activation of multiple lamps.
Accordingly, the present invention solves the drawbacks of the prior art by providing a sequential burst mode activation circuit for multiple loads by generating a phase shift between multiple burst-mode signals. The burst mode signals are used to regulate power delivered to loads, where each load is regulated by a separate phase-shifted burst signal such that at least two loads do not turn on synchronously. The circuit of the present invention overcomes prior art regulation circuits by eliminating instantaneous high current ripples and noise created by multiple loads turning on simultaneously.
The present invention provides a sequential burst mode activation circuit comprising a variable power regulator, comprising a pulse modulator generating a pulse signal having a pulse width; a frequency selector generating a frequency selection signal; and a phase delay array receiving said pulse signal and said frequency selection signal, and generating a plurality of phased burst signals, wherein at least two of said phased burst signals have different start times.
In method form, the present invention provides a method for generating phase shifted burst mode signals, comprising the steps of generating a pulse signal having a pulse width, generating a frequency selection signal, generating a plurality of phased burst signals having a frequency of said frequency selection signal and pulse width of said pulse signal, and delaying at least one of the phased burst signals to have a different start time than at least one other of the phased burst signals.
The present invention also provides a phased burst mode dimming system, comprising: a pulse width modulator generating a pulse width modulated signal; a variable selector for selecting the width of said pulse width modulated signal; and a phase delay array receiving said pulse modulated signal and said frequency selection signal, and generating a plurality of phased burst signals by generating a phase delay between at least two said pulse width modulated signals.
In one exemplary embodiment, power is regulated to a plurality of loads using the plurality of phased burst signals. Additionally, a constant or variable phase delays is generated between each phased burst mode signal. In an exemplary system, the present invention provides a sequential burst mode dimming circuit for multiple lamps. In particular, the exemplary system provides a sequential burst mode dimming circuit for a plurality of cold cathode fluorescent lamps (CCFLs). Customer or software inputs vary the pulse width of a PWM signal, thereby determining the power to be delivered to the lamps. A reference signal is doubled to select the frequency of the PWM signal. This selected frequency determines the frequency at which lamps turn on and off. Using a counter and a clock, multiple phased burst signals are generated from the above burst signal for the plurality of CCFL""s. Each phased burst signal is shifted by a constant phase shift such that at least two lamps receive burst signals that are out of phase. Therefore, sequential burst-mode activation of each lamp is generated. Finally, in the exemplary system, a plurality of phase array drivers, each of which uses feedback from a corresponding lamp in combination with a corresponding phased burst signal, delivers power to and regulates the intensities of a corresponding plurality of lamps.
Another exemplary system of the present invention includes a frequency selector that generates a frequency selection signal for a backlight load which follows, as reference, a conventional screen updating frequency of a cathode ray tube (CRT) in a television set. In yet another exemplary system, a phase delay array generates a plurality of phased burst signals, such that no two phased burst signals have different start times. In an example of such an embodiment, a phase delay array generates a constant or variable phase delay so that each of the phased burst signals is delayed by such a phase delay from another of the phased burst signals.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to exemplary systems and methods of use, the present invention is not intended to be limited to these exemplary systems and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: